The present invention relates to web supporting and drying apparatus. In drying a moving web of material, such as paper, film or other sheet material, it is often desirable to contactlessly support the web during the drying operation in order to avoid damage to the web itself or to any ink or coating on the web surface. A conventional arrangement for contactlessly supporting and drying a moving web includes upper and lower sets of air bars extending along a substantially horizontal stretch of the web. Heated air issuing from the air bars floatingly supports the web and expedites web drying. The air bar array is typically inside a dryer housing which can be maintained at a slightly sub-atmospheric pressure by an exhaust blower that draws off the volatiles emanating from the web as a result of the drying of the ink thereon, for example.
One example of such a dryer can be found in U.S. Pat. No. 5,207,008, the disclosure of which is hereby incorporated by reference. That patent discloses an air flotation dryer with a built-in afterburner, in which a plurality of air bars are positioned above and below the traveling web for the contactless drying of web coating. In particular, the air bars are in air-receiving communication with an elaborate header system, and blow air towards the web so as to support and dry the web as it travels through the dryer enclosure.
Various attempts have been made in the prior art for decreasing the length and/or increasing the efficiency and line speed of such dryers. To that end, infrared radiation has been used either alone or in combination with air to dry the web. However, installing infrared radiation means in conventional convection dryers is often difficult and the equipment is expensive to purchase and to operate.
U.S. Pat. No. 4,698,914 discloses a dryer having a series of sections; each section having at least one push-type and one draw-type gas discharge device, such as an air bar and an air foil, respectively. The push-type device is arranged so as to cause gas to impinge the side of the web opposite the coated side and at an angle of substantially 90.degree. relative to the transport direction of the web. The draw-type device is arranged so as to cause gas to impinge the side of the web opposite the coated side at an angle of about 0.5.degree. to 5.0.degree. relative to the transport direction of the moving web. As a result, web clearance is increased and web defects reduced.
U.S. Pat. No. 3,979,038 discloses a flotation dryer including a plurality of blow boxes provided with apertures for air outflow against a floating web, and fixing chambers mounted at a smaller distance from the web than the blow boxes. The fixing chambers have apertures directed obliquely to the plane of the web, and at least one blow box with apertures distributed over its plane is mounted directly in front of a fixing chamber.
The present invention relates to a web flotation dryer and a process for floatingly drying a traveling web, wherein a combination of air bars and hole bars are used. Although more nozzles may be used overall in the present invention, less air bars are used. This is advantageous in view of the precise tolerance that air bars require, which add to their cost of manufacture. The use of hole bars also allows for a reduction in power requirements and operation at lower nozzle velocities without sacrificing heat transfer efficiency, and indeed, in some instances, enhancing heat transfer.
It is therefore an object of the present invention to improve the heat transfer process in an air flotation dryer without substantially increasing the capital or operating costs.
It is a further object of the present invention to achieve efficient heat transfer using the same or less total air volume per unit drying area than in a conventional dryer.
It is a still further object of the present invention to achieve efficient heat transfer while using lower air horsepower for a given heat transfer coefficient.